1. Field of the Invention
This invention pertains to processing insulated electrical conductors, and more particularly to apparatus and methods for removing selected portions of insulation from electrical conductors.
2. Description of the Prior Art
Various equipment has been developed to strip insulation from insulated electrical conductors and to cut long lengths of conductor into individual pieces. Examples of such equipment may be seen in U.S. Pat. Nos. 4,577,405; 4,630,406; and 4,702,136. The tooling and machinery illustrated in the foregoing patents are useful primarily for stripping selected lengths of insulation from the ends of discrete pieces of insulated wire.
Another very important aspect of processing insulated wire is the removal of short lengths of insulation at one or more locations intermediate the wire ends. The removal of discrete portions or slugs of insulation from the interior of a length of wire is referred to as center notching. Such terminology is somewhat misleading since a notch formed by removing a slug of insulation need not be at or even near the center of a piece of insulated wire. In fact, several slugs may be removed from a single piece of wire. However, to maintain consistency with industrial usage, the term center notching as used herein is defined as the removal of one or more pieces or slugs of insulation from anywhere between the ends of a length of insulated electrical conductor.
Center notching is highly desirable in electrical circuits that require the stripped ends of separate lengths of insulated wires to be spliced or doubled together to form an electric wire harness. As an example, without center notching, a three-legged harness requires five components, three wires and two terminals, to connect the ends of the three wires together. A simple three-wire harness 2 is pictured in FIG. 12a. Wires 4 and 6 are doubled on one side of the joint 8. Both of the wires 4 and 6 are spliced to wire 10 by means of a female terminal 12 and a male terminal 14. An exploded view of the components of the harness 2 is shown in FIG. 12b.
With center notching, one wire length is eliminated, since two legs of the harness are replaced by one center notched length. The third leg is attached to the center notch, as is shown in FIG. 13a. Therefore, in this example, only four components, one center notched wire 16, one other wire 18, and two terminals 20 and 22, are required Numerous designs of terminals are available for that type of connection.
A further simplification can be realized if the third leg of the harness is welded to the wire center notch, thereby eliminating the terminals.
Referring to FIG. 13b, a harness 30 is shown that has only two components. They are a center notched wire 24 and another wire 26 that are joined together by a weld 28. The harness 30 represents a substantial reduction in the number of components compared with the harness 2 of FIGS. 12a and 12b. The ability to reduce the number of harness components offers cost savings not only in piece parts and material handling, but also in increased reliability due to fewer parts that could potentially fail.
To date, center notching of insulated electrical conductors has not been performed in a satisfactory manner. Prior apparatus and methods lack the ability to remove slugs of insulation on an automated basis. One method of center notching, and the tooling used therefore, is illustrated in FIG. 1. A pair of blades 1 and 3 have respective radius cutting edges 5 and 7 that conform to the circular periphery of the conductor 9 of an insulated electrical conductor 11. The blades 1 and 3 are closed in the direction of arrows 13 over the insulated electrical conductor 11 to slice the insulation 15 around the periphery of the conductor 9. With the blades in the closed position of FIG. 1b, the wire 11 is pulled axially in the direction of arrow 17, with the blades remaining fixed. As a result, a short length 19 of conductor 9 is exposed. The short length 19 of bare conductor represents the intended center notch. It will be apparent that the center notch is produced by sliding or compressing the insulation along the conductor rather than by stripping a slug of unwanted insulation from the wire.
Other variations of prior center notching techniques include the use of two pairs of blades, such as blades 1 and 3. The blades are placed with their respective pairs of cooperating cutting edges next to each other. When closed over an insulated wire 11 the two pairs of cutting blades essentially make a single circumferential slice in the insulation. The two pairs of blades are then translated in opposite directions to each other along the axis of the insulated electrical conductor. Each pair of blades axially compresses the insulation in the direction of the blade axial motion. As a result, bare conductor 9 is exposed between the two sets of blades. Whether one or two pairs of blades are used, the procedure of compressing insulation rather than removing a discrete slug of it is a major disadvantage of prior center notching methods
Since no insulation is actually removed from an insulated electrical conductor by prior methods and equipment, the length of a notch 19 can vary significantly from the desired length. Variables that affect actual notch length include the elasticity and compressibility of the insulation, the bonding between the insulation and the conductor, the twist of lay of the conductor strands, the spacing between notches, and ambient conditions. The longer the notch length, the more the insulation must be compressed. Many insulation materials such as plastics and rubber have memories. Consequently, after they have been compressed, they tend to relax back to their former condition when the compressive force is removed. Prior center notching is based on the assumption that the compressed insulation does not relax significantly. Such an assumption is not always justified in practice, with the result that prior center notched wires are unreliable.
Accurately controlling the length of the center notch is very important in applications in which a terminal is to be assembled to the exposed conductor. Typical terminals have conductor ears or tabs that are crimped onto the conductor 9. It is imperative that no insulation get between the terminal and conductor. With prior center notching, there is an undesirably high probability that the compressed insulation will relax prior to terminal attachment, thereby causing an improper assembly and impairing conductivity. Further, many terminals have insulation ears that are designed to crimp around adjacent parent insulation for added pull test strength. Thus, variations in notch lengths create problems in locating and attaching terminals, and the quality of the resulting harness is suspect.
Thus, a need exists for improved methods and equipment for center notching insulated electrical conductors.